Hermatic firewall for MEMS packaging in flip-chip bonded geometry

ABSTRACT

A package for hermetically sealing a micro-electromechanical systems (MEMS) device in a hybrid circuit comprise a firewall formed on a substrate for the MEMS device and which has a height defining a cavity of the package in which the MEMS device will be sealed. A second substrate spaced from the first substrate hermetically seals the cavity when the second substrate is flip-chip bonded to the first substrate and soldered to the first substrate with a thin film metal material placed on at least a top portion of the firewall. The resulting firewall MEMS device package can be further packaged using conventional CMOS packaging techniques. By hermetically sealing the cavity, the enclosed MEMS device is protected from deleterious conditions found in the environment of conventional CMOS packaging techniques which is often detrimental to MEMS device function.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to packaging for micro-electromechanicalsystems (MEMS) devices. More specifically, the present invention relatesto packaging of MEMS devices in flip-chip bonded geometry to providehermetic seals for the MEMS devices.

2. Description of the Related Art

MEMS devices have become ubiquitous in the semiconductor industry andare used in hybrid electrical and mechanical functions that arenecessary in many electronic and electro-optical systems. Packaging MEMSdevices in such systems is in general a costly process. Becausemechanical motion is an essential part of the MEMS function, a carefullydesigned space for the motion is needed in order for the MEMS device tooperate reliably. Conventional packaging techniques used for CMOSelectronics are performed after the CMOS devices are appropriatelyprotected using proper passivation material. Such passivation procedureprotects the devices from the packaging processes that are usuallymechanically and chemically harsh. After device passivation, packagingis performed by injecting either molten plastic or epoxy over the CMOSchip. Since the CMOS function relies purely on the electronic propertiesof silicon, such a packaging process does not interfere with deviceperformance. However, these packaging techniques usually involvetreatment of semiconductor chips using a fluid, and are therefore notcompatible with MEMS device operation. For this reason, expensiveceramic packaging has usually been chosen over cost-effective plasticmolded packages for MEMS devices.

Most MEMS devices need to operate in a controlled environment to achieveoptimum device performance and reliability. Some examples of controlledenvironments include controlled pressure (vacuum), controlled humidity,and controlled chemical (typically a special gas) environments.Packaging MEMS devices cost-effectively under these conditions isdesired in the art, but due to the complexities and sensitivitiesassociated with operation environments of MEMS devices it is often adifficult or impractical task.

It is also essential that such cost-effective MEMS packaging technologybe compatible with CMOS packaging technology. This is important becauseCMOS technology is already mature and commercially available. Moreover,the need to integrate MEMS devices with CMOS technology is becomingincreasingly important in order to add functionality to the CMOS chipthat simple CMOS devices cannot provide, and to provide control of MEMSdevices using CMOS integrated circuits. Most CMOS packaging techniqueswherein wet chemistry and high-pressure fluid flows are used, however,are detrimental to MEMS devices. Thus, current CMOS packaging techniqueswill not adequately protect the MEMS devices during the packagingprocess.

Accordingly, there is a long-felt, but unresolved need in the art forMEMS packaging techniques which hermetically seal the MEMS devices sothat they can effectively be incorporated in CMOS and other hybridcircuits. The packages should be cost-effective and ensure thatelectrical connections to or with the MEMS devices can be achievedwithout breaching the integrity of the package. Moreover, such packagesshould be easily integratable with current semiconductor fabricationprocesses and be compatible with conventional CMOS packaging methods.

SUMMARY OF THE INVENTION

The present invention provides a novel packaging technique that enablesconventional CMOS packaging procedures be used to package MEMS devices.It also provides means to control the operation environment of the MEMSdevices at the same time. The inventive packages produce a protectedcavity around the MEMS devices which is created by a a flip-chip bondingprocess. In a preferred embodiment, a firewall is fabricated on a firstsubstrate around the MEMS device to enclose the MEMS device within thecavity bounded by the firewall. Another substrate is then flip-chipbonded to the first substrate that holds the MEMS device. This secondsubstrate may hold other MEMS devices to complete the structure orfunctionality of the overall hybrid circuit, may hold CMOS electronicsto control the MEMS device(s) on the first and/or second substrate, ormay serve purely as a mechanical “cover” of the MEMS device firewall. Agap between the two substrates is accurately controlled by the height ofthe firewall itself, by spacers of known height, or with spacers inconjunction with the height of the firewall. The spacers can befabricated independently on the substrate, in which case they will notform a portion of the firewall per se. In a preferred embodiment, thecavity is hermetic, which means that the cavity is sealed against theenvironment of the package to protect the MEMS device from anydeleterious conditions found or present in the environment of thepackage.

Upon flip-chip bonding of the two substrates, the firewall seals off thespace immediately around the MEMS device(s). At the same time,mechanical support and integrity is provided for the package by thebonded substrates through appropriate bonding techniques. In still afurther preferred embodiment, the hermetic firewall itself provides themechanical support for the package. Still more preferably, independentstructures are provided to the package to give the package itsadditional mechanical support. Once the cavity has been created by thefirewall and the MEMS device is protected accordingly, the hybrid chipcontaining the MEMS device in its package can be further packaged usingconventional CMOS packaging technology.

The inventive packages for MEMS devices are simple to implement and caneasily be performed with conventional CMOS packaging technology.Moreover, packages provided in accordance with the present invention mayhermetically seal MEMS devices from deleterious effects found in apackaging environment which could damage the MEMS devices. Thus, thepackages disclosed and claimed herein efficiently protect MEMS devicesso that these devices can function robustly when in use.

These and other features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are designed solely for purposes of illustration and not as adefinition of the limits of the invention, for which reference should bemade to the appended claims.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are designed solely for purposes of illustration and not as adefinition of the limits of the invention, for which reference should bemade to the appended claims. It should be further understood that thedrawings are not necessarily drawn to scale and that, unless otherwiseindicated, they are merely intended to conceptually illustrate thestructures and procedures described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, wherein like reference numerals identify similarelements throughout the several views:

FIG. 1 is a schematic, cross-sectional view of a hermetic firewallpackage for enclosing MEMS devices in accordance with the presentinvention;

FIG. 2 is an elevated perspective view of a hermetic firewall MEMSpackage wherein the firewall serves as a spacer while mechanical supportis provided by solder bumps outside of the firewall;

FIG. 3 is a perspective view of a hermetic firewall MEMS package of theinvention wherein the firewall includes a ring-shaped solder seal thatprovides the mechanical support and spacers are provided separately;

FIGS. 4a-c depict a schematic process of solder bump or ring-shapedsolder seal; and

FIG. 5 is an elevated perspective view of a hermetic firewall MEMSpackage of the invention wherein a double-walled structure isimplemented and wherein an inner wall provides the MEMS sealedenvironment while an outer wall includes a ring-shaped solder seal thatprovides a second hermetic seal and mechanical support for the package.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 1 depicts a schematiccross-sectional view of the hermetic firewall structure of the presentinvention identified by the general reference numeral 10. The package 10comprises a cavity 20 for enclosing a MEMS device 25 or several MEMSdevices, depending on the particular hybrid circuit application in whichthe MEMS device(s) will be integrated. In a preferred embodiment, afirewall 30 is fabricated on one or both substrates 40 and 70 on one ofwhich the MEMS device is fabricated. The firewall 30 includes a topsurface 50 and a bottom surface 60. The package 10 further comprises asecond substrate 70 which is bonded to the first substrate through thefirewall 30 formed on substrate 40. The second substrate 70 contains amating seal to the firewall 30, which completes the firewall structure.Preferably, the second substrate is “flip-chip” bonded to substrate 40,using a suitable bonding technique. A preferred bonding technique is aheat-based process wherein a material such as a metal or plastic resinis placed between two parts to be welded together and heated to melt orsoften the material. When the material thereafter hardens, a strong,resilient seal is created between the two pieces. Substrates 40, 70 areconventional substrates used to fabricate CMOS electronic devices. Suchsubstrates usually comprise silicon, although it will be recognized bythose skilled in the art that the substrates may, for example, compriseGaAs, Ge or other semiconductor materials, or insulating materials suchas quartz, alumina, or sapphire. For ease of description hereinbelow butwithout intending to limit the invention, it will be assumed that thesubstrates are silicon substrates.

The firewall 30 is fabricated around the MEMS device 25, and may befabricated on either of the substrates 40, 70. The height of thefirewall 30 can be precisely controlled to thereby control the spacingbetween the two substrates. For some MEMS devices, the spacing may be anintegral part of the MEMS device function, while in others it has to besimply large enough to accommodate the MEMS devices in the cavity 20. Asealing material is employed atop the firewall 30 to produce a hermeticseal 80 for the cavity 20. The sealing material is preferably a thinfilm metal material and is placed on at least a portion of the top 50 offirewall 30 to seal the cavity 20 when the second substrate 70 isflip-chip bonded to first substrate 40. During the flip-chip bondingprocess, the welding materials on both sides of the substrates areheated and pressed together, resulting in a tight hermetic seal betweenthe flip-chip bonded substrates 40, 70. After the flip-chip bondingprocess, cavity 20 is bounded and formed by the firewall 30 andsubstrates 40, 70 and encloses a small space around the MEMS device 25.This leaves the MEMS device 25 intact and protected from subsequentpackaging processes performed outside cavity 20, which might involvefluid treatment that have deleterious effects on the MEMS device 25.

The hermetic firewalls of the present invention can be fabricated usingthe same process that is employed for fabricating MEMS devices. In anexample of MEMS devices fabricated using silicon surface micromachiningtechnology, the firewalls may be made up of alternating stacks ofpolycrystalline silicon and silicon dioxide, encapsulated bypolycrystalline silicon. Another alternative is to use a material thatis deposited or spin-coated and patterned on a substrate using alithography technique. Some examples include silicon nitride, polyimideand metal. In all cases, the material comprising the firewall 30 musthave the desired chemical and mechanical strength to create a hermeticseal between the substrates and the firewall and to protect the MEMSdevice 25. Using such techniques, the height of the walls can be definedprecisely.

In a preferred embodiment depicted in FIG. 2, a hermetic seal 80 betweenthe firewalls on first and second substrates can be formed byevaporating a soft metal such as gold, silver, or their alloys onto thetop of firewall 30, and pressing down on the metal surface in a heatedenvironment. The heated environment may be created during the flip-chipbonding process of the first substrate to the second substrate itself,or by an independent heating or soldering process known to those skilledin the art. However, creating such a seal 80 usually does not provideenough mechanical strength to hold the substrates together and toprovide mechanical integrity to the package 10. A stronger mechanicalseal may be achieved using stronger solder, which is used inconventional flip-chip bonded packaging techniques. When it is desiredto use additional solder to provide a stronger mechanical support, aplurality of solder bumps 90 can be bonded to the first substrate 40which will provide mechanical support and integrity to the package, whenthe second substrate (not shown in FIG. 2) is flip-chip bonded to thefirst substrate. Alternately, the solder bumps 90 may be bonded to thesecond substrate which is simply a matter of design choice.

Of critical importance in the fabrication of packages for MEMS devicesin accordance with the present invention is that the electricalconnection to the MEMS device 25 with the rest of the circuit musttraverse firewall 30 without breaching the hermatic seal enclosing thecavity 20. In a preferred embodiment, electrical leads 100 connectingthe MEMS device 25 to the rest of the circuit are inserted throughfirewall 30 during the MEMS fabrication process. In this case, forexample, the leads can be made up of heavily doped conductivepolycrystalline silicon, encapsulated by silicon dioxide layers toachieve electrical isolation. In another preferred embodiment, theelectrical leads 100 can be placed underneath the firewall.Alternatively, leads 100 may be secured to the second substrate 70 thatcontains CMOS circuitry, for example, thereby alleviating the need tobreach or otherwise corrupt the firewall 30 by requiring the leads 100to physically traverse therethrough. In this case, vertical electricalconnection between the MEMS device 25 on substrate 40 and the electricalleads on substrate 70 has to be fabricated. Such connection can easilybe achieved by means of solder bumps or metallizing independent spacers120 (FIG. 3).

In the embodiment of FIG. 2, the firewall 30 comprises stacks ofpoly-silicon and silicon dioxide layers. The firewall 30 itself alsoserves as a spacer for cavity 20 to control the gap between the twosubstrates 40 and 70 so that cavity 20 has a height defined by theheight of firewall 30. Mechanical strength is provided by independentsolder bumps 90 placed outside the firewall 30. The solder bumps 90 maybe placed at any convenient location on either or both of substrates 40,70. The electrical leads 100 preferably comprise poly-silicon surroundedby silicon dioxide layers, encapsulated by another layer ofpoly-silicon.

FIG. 3 depicts yet another preferred embodiment of the package 10 of thepresent invention having a hermetic firewall 30. A metal film asdescribed above is evaporated on top 50 of the firewall structure andwill form the hermetic seal 80 for the cavity 20. The solder material isdeposited on top 50 of firewall 30 to produce a solder seal 110 forcavity 20. When the solder seal 110 is made, it provides both thehermeticity and mechanical strength necessary for the flip-chip bondingprocess to produce a sturdy, strong hermetic package. Independentspacers 120 may also be provided to accurately define the required gapor spacing between the substrates. Spacers 120 may be fabricated oneither or both of substrates 40, 70 and may be placed at arbitrarylocations thereon.

The process of solder bump or ring-shaped solder seal is schematicallyshown in FIGS. 4a-4 c. In all preferred embodiments utilizing solderbump or ring-shaped solder seal bonding, the height of the spacers 120are intentionally made higher than the solder bumps. For the solder bumppreparation, one can deposit solder 75 onto a larger footprint thanmetal pad, over a dielectric layer 85 that the solder does not wet (FIG.4a). Upon heating, the surface tension will increase the solder bump 90height and decrease its footprint (FIG. 4b). This mechanism enables thesolder bumps 90 to make contact to the mating metal pad 95 on the othersubstrate. Upon cooling, the solder shrinks and actively pulls the twosubstrates together (FIG. 4c). This process guarantees an intimateconnection between the spacers, and precise separation between the twosubstrates.

One disadvantage of the embodiment of FIG. 3 is that if any fluxmaterial (either in the gas phase or the liquid phase) is necessary forsoldering, the MEMS device 25 will be exposed to this environment. Thismight not be detrimental to MEMS device 25 if the proper flux is used,but will limit the ability to provide a controlled environment for theMEMS device.

FIG. 5 depicts still another preferred embodiment of the package 110containing a hermetic firewall of the present invention. In thisembodiment, a double-walled firewall structure is fabricated on eithersubstrate 40, 70. The inner wall 130 is similar to that of FIG. 1wherein metal layers 80 are eposited on the top 50 of firewall 30, andprovide the appropriate spacing for cavity 20. An outer wall 140 issimilar to the firewall 30 of FIG. 3 wherein a layer of solder material110 is deposited on the top 50 of firewall 30. The mechanical strengthfor the resulting package is provided by the outer wall 140. In thisembodiment, the inner wall 130 is bonded to either substrate 40, 70 witha “tack” bond between two metal layers 80 that is created by applyingpressure between the two substrates at a low temperature as compared tothe melting temperature of the solder of seal 110. When this process isaccomplished in a controlled environment, the space encapsulated by theinner wall 130 maintains this environment and is sealed off. After thelow-temperature tack bonding is accomplished, it is possible to simplyheat up the substrate 40 or 70 to a temperature sufficient to melt thesolder to thereby form the solder bond on outer wall 140. Thedouble-walled firewall of FIG. 4 thus advantageously protects MEMSdevice 25 from the deleterious effects of the soldering process.

The environment within the cavity defined by the firewall can becontrolled by performing the flip-chip bonding process under the desiredenvironment. Such desired environment may include, for example,controlled pressure, controlled humidity, and controlled gaschemistries. The cavity 20 created by this flip-chip bonding processprotects the MEMS devices, so that the flip-chip bonded substrates cannow be further packaged using conventional packaging techniques thatmight otherwise be detrimental to the MEMS devices.

While there have been shown and described certain fundamental novelfeatures of the invention as applied to preferred embodiments thereof,it will be understood that various omissions and substitutions andchanges in the methods and apparatus described herein, and in theiroperation, may be made by those skilled in the art without departingfrom the spirit and scope of the invention. It is expressly intendedthat all combinations of those elements and/or method steps whichperform substantially the same function in substantially the same way toachieve the same result are within the scope of the invention.Substitution of elements from one described embodiment to another arealso fully intended and contemplated. It is the intention, therefore, tobe limited only as indicated by the scope of the claims appended hereto.

Thus, while there have shown and described and pointed out fundamentalnovel features of the invention as applied to a preferred embodimentthereof, it will be understood that various omissions and substitutionsand changes in the form and details of the devices illustrated, and intheir operation, may be made by those skilled in the art withoutdeparting from the spirit of the invention. For example, it is expresslyintended that all combinations of those elements and/or method stepswhich perform substantially the same function in substantially the sameway to achieve the same results are within the scope of the invention.Moreover, it should be recognized that structures and/or elements and/ormethod steps shown and/or described in connection with any disclosedform or embodiment of the invention may be incorporated in any otherdisclosed or described or suggested form or embodiment as a generalmatter of design choice. It is the intention, therefore, to be limitedonly as indicated by the scope of the claims appended hereto.

What is claimed is:
 1. A packing having a cavity with a height forhaving a micro-electromechanical systems (MEMS) device and sealing theMEMS device in the cavity to protect the MEMS device against deleteriousconditions present in an environment of the package, comprising: a firstsubstrate for defining a base on which the MEMS device is fabricated; afirewall fabricated on the first substrate and having a bottom surfaceengaged with the first substrate, said firewall forming walls of thecavity which extend upwardly away from the bottom surface for fullysurrounding the MEMS device within the cavity which is bounded andformed by the firewall and the first substrate, said firewall comprisingalternating stacks of poly-silicon and silicon dioxide encapsulated bypoly-silicon; and a second substrate bonded to the first substrate overthe firewall and in sealed engagement with the firewall for creating acavity-closing seat for the cavity within which the MEMS device is fullyenclosed against the deleterious conditions present in the environmentof the package so that the MEMS device is packaged protectedly withinthe cavity and remote from the deleterious conditions.
 2. The package ofclaim 1, wherein the seal is a hermetic seal.
 3. The package of claim 2,wherein the hermetic seal comprises a thin film metal materialoverlaying at least a portion of a top surface of the firewall.
 4. Thepackage of claim 3, wherein the hermetic seal comprises a thin filmmetal material soldered to the top surface of the firewall to producethe hermetic seal having a mechanical strength and imparting structuralintegrity to the package.
 5. The package of claim 4, further comprisinga plurality of solder bumps fabricated on the first substrate tosupplement the mechanical strength provided by the hermetic seal.
 6. Thepackage of claim 5, wherein the thin film metal material comprises gold.7. The package of claim 5, wherein the thin film metal materialcomprises silver.
 8. The package of claim 5, wherein the thin film metalmaterial comprises an alloy of gold.
 9. The package of claim 5, whereinthe thin film metal material comprises an alloy of silver.
 10. Thepackage of claim 1, further comprising electrical leads connected to theMEMS device in the cavity and extending through the firewall.
 11. Thepackage of claim 10, wherein the electrical leads comprise apoly-silicon layer surrounded by a silicon dioxide layer which isfurther surrounded by another layer of poly-silicon.
 12. The package ofclaim 1, further comprising a plurality of spacers formed on either oneor both of the two substrates to space apart the two substrates by apredetermined distance and thereby define a height of the cavity, andwherein the second substrate is placed over the cavity in further sealedengagement with the plurality of spacers.
 13. A package having a cavitywith a height for housing a micro-electromechanical systems (MEMS)device and sealing the MEMS device in the cavity to protect the MEMSdevice against deleterious conditions present in an environment of thepackage, comprising: a first substrate for defining a base on which theMEMS device is fabricated; a firewall fabricated on the first substrateand having a bottom surface engaged with the first substrate, saidfirewall forming walls of the cavity which extend upwardly away from thebottom surface, for fully surrounding the MEMS device within the cavitywhich is bounded and formed by the firewall and the first substrate;electrical leads connected to the MEMS device in the cavity andextending through the firewall, said electrical leads comprising apoly-silicon layer surrounded by a silicon dioxide layer which isfarther surrounded by another layer of poly-silicon; and a secondsubstrate bonded to the first substrate over the firewall and in sealedengagement with the firewall for creating a cavity-closing seal for thecavity within which the MEMS device is fully enclosed against thedeleterious conditions present in the environment of the package so thatthe MEMS device is packaged protectedly within the cavity and remotefrom the deleterious conditions.
 14. The package of claim 13, furthercomprising a plurality of spacers formed on either one or both of thetwo substrates to space apart the two substrates by a predeterminedamount and thereby define a height of the cavity, and wherein the secondsubstrate is placed over the cavity in further sealed engagement withthe plurality of spacers.
 15. The package of claim 13, wherein the sealis a hermetic seal comprising a thin film metal material overlaying atleast a portion of a top surface of the firewall.
 16. The package ofclaim 15, wherein the hermetic seal is soldered to the top surface ofthe firewall to produce the hermetic seal having a mechanical strengthand imparting structural integrity to the package.
 17. The package ofclaim 16, further comprising a plurality of solder bumps fabricated onthe first substrate to supplement the mechanical strength provided bythe hermetic seal.
 18. The package of claim 13, wherein the firewallcomprises alternating stacks of poly-silicon and silicon dioxideencapsulated by poly-silicon.